I have a regular computer with CORE I3, according to information here, it's said that it's very difficult to mine any coin.
Is it true?
You will barely get a few cents and spend a lot more in energy by using just a i3. If you really want to mine altcoins and earn money, you will probably need to buy a few GPU's.
For example, if you mine at Nicehash with a i7-7700K (the one I have), you will end up earning only ~$40 per month (minus your energy bill). And that's not very profitable.